Lunging with a simultaneous twist helps to build not only leg and core strength but adds balance and stability.
The "secret" is to keep your core tight and rotate your upper torso in a smooth controlled manner keeping the weight level.
Variations of this exercise include lunging forward & lunging backwards. If lunging forward, return to upper position by driving backwards off front foot. If lunging backwards, return to upper position by driving "upwards" with front leg.
Another variation of this exercise is to to perform while lunging over a set distance alternating legs.